1. Field of the Invention
The present invention generally relates to a chemical mechanical polishing (CMP) process, and more particularly to a dummy process and a polishing-pad conditioning process suitable for a CMP apparatus.
2. Description of Related Art
With continuous reduction in size of semiconductor devices, resolution of photolithographic exposure in semiconductor fabrication processes has been enhanced, and consequently, with decrease of depth-of-focus of exposure, requirements on smoothness of the surface of wafers has accordingly become more stringent. Typically, planarization of wafers is usually accomplished in a CMP process. CMP process have a distinctive feature of anisotropic polishing and thus is suitable not only for planarization on outer surface of wafers but also for fabrication of inlay structures of metal interconnects, shallow trench isolation, micro-mechanical and electrical systems, and plane monitors.
In a CMP process, a wafer is usually fixed on a polishing head with the surface of the wafer to be polished facing the rotating polishing pad. Next, polishing slurry consisting of abrasive particles and chemical agents is provided on the polishing pad. Next, the polishing head is then made to come in contact with the wafer surface, and a suitable pressure is applied onto the wafer to press the wafer firmly on the polishing pad. The polishing of the wafer surface effected by the chemical reactions between the wafer surface and the polishing slurry and the mechanical abrasive action between wafer surface, the abrasive particles of the polishing slurry and the polishing pad. Thus, a CMP process to obtain a planar surface with smooth topography.
FIG. 1 shows relative position and interaction between a polishing head and wafer in a conventional CMP process. Typically, a conventional polishing head 100 includes a protective hood 102, a base 104, a retaining ring 106, and a wafer supporting assembly 108. The base 104 is below the protective hood 102, the wafer supporting assembly 108 is below the base 104, and the retaining ring 106 is fixed around the rim of the base 104. Wherein, a wafer supporting assembly 108 includes a supporting board 110 and a membrane 112 connecting the supporting board 110, while the membrane 112 connects with and extends under the supporting board 110 to provide an attaching surface 114 for the wafer 10.
During the CMP process, a wafer 10 is placed under the attaching surface 114 and a pressure is applied onto the base 104 so that the wafer supporting assembly 108 moves down to firmly contact with the wafer 10. Next, the wafer 10 held by the polishing head 100 is moved onto a surface of the polishing pad 122 attached to a polishing table 120. Next, the wafer and the polishing pad are rotated relative each other such that the wafer is held against the surface of the polishing pad under pressure. In other words, the hollow chamber 116 between the base 104 and the wafer supporting assembly 108 is under pressure, and consequently, an upward force 132 is applied onto the base 104 and a downward force 134 is applied onto the wafer supporting assembly 108. Hence, the downward force 134 on the wafer supporting assembly 108 will press the wafer 10 onto the polishing pad 122. Then, the polishing table 120 can be rotated relative to the polishing head 100 for planarizing the wafer surface.
When a CMP polisher is idle, a dummy process is usually performed to keep the polishing pad under a stable condition. The dummy process is identical to a CMP process (shown in FIG. 1) except the wafer is replaced with a dummy wafer. Therefore, a large number of dummy wafers are required for maintaining a conventional CMP apparatus, and these dummy wafers must be coated with a membrane prior to its use in a dummy polishing process.
Apparently, a large number of dummy wafers are required, which would undoubtedly increase the costs of the already expensive CMP process.